Fix ArgNotSet repr to use stable string instead of memory address (apache#65222)

* Add tests for ArgNotSet serialization and repr/str behavior

* Add serialize, __repr__, __str__ to ArgNotSet for deterministic serialization

* Flip ArgNotSet repr/str test to expect sentinel string and use top-level imports

* Delegate ArgNotSet __repr__/__str__ to serialize() for polymorphic sentinel strings

---------

Co-authored-by: taeyong <taeyong4.kim@navercorp.com>

Author: 2 people

task-sdk/src/airflow/sdk/definitions/_internal/types.py

@@ -40,6 +40,12 @@ def is_arg_passed(arg: Union[ArgNotSet, None] = NOTSET) -> bool:
     def serialize():
         return "NOTSET"
 
+    def __repr__(self) -> str:
+        return self.serialize()
+
+    def __str__(self) -> str:
+        return self.serialize()
+
     @classmethod
     def deserialize(cls):
         return cls